The olefin metathesis reaction is a highly versatile and powerful technique for the synthetic preparation of alkenes. Transition metal carbene complexes—particularly those incorporating ruthenium—are popular catalysts for metathesis. However, the yield of certain desired metathesis products can be significantly reduced by double bond isomerization. This is typically the result of residual metathesis catalyst and/or its byproducts being present in the reaction mixture. This problem becomes particularly acute if the metathesis mixture is heated and/or distilled in the presence of residual catalyst.
In view of this problem, it is oftentimes necessary to remove residual metathesis catalyst from an olefinic metathesis product (or otherwise passivate the residual catalyst) prior to subjecting the olefinic metathesis product to further chemical reactions and/or processing. One approach, as described in U.S. Pat. No. 6,215,019 B1, has been to add tris(hydroxymethyl) phosphine (THMP) to the reaction mixture as an isomerization inhibitor. Unfortunately, the commercial availability and pricing of THMP are not viable on an industrial scale. Moreover, although THMP can be prepared from precursor salts, such as tetrakis(hydroxymethyl) phosphonium sulfate (THPS) or tetrakis(hydroxymethyl) phosphonium chloride (TKC), the conversion involves generation of formaldehyde—a known human carcinogen—as a byproduct. In addition, if pH is not strictly controlled during the formation of THMP (e.g., if conditions become too basic), explosive hydrogen gas has been known to form.
An isomerization suppression agent that efficiently passivates residual metathesis catalyst present in admixture with olefinic metathesis product, and which is readily available on a commercial scale but does not produce carcinogenic by-products and/or involve the formation of explosive hydrogen gas is needed.